1. Field of the Invention
The present invention broadly relates to optical defects in transparencies and, more particularly, is concerned with a dynamic distortion recording method utilizing photographic time exposure techniques to record the "distortion track" of a test target pattern observed through a moving transparency, such as an aircraft windscreen.
2. Description of the Prior Art
As a general rule, optically transparent, asymmetrically contoured bodies have been difficult to quantitatively evaluate and compare on the basis of their optical characteristics. A prime example of a structural element formed of a transparent medium in which optical quality is critical, yet difficult to quantitatively evaluate, is the canopy or windscreen of aircraft having complex curvilinear contours.
Distortion is one of the optical quality parameters that has been identified for characterizing transparencies, such as aircraft windscreens. Distortion is the non-linear mapping of object points to image space due to the optical effects of the transparency. Such effects may be due to either optical index variations in the transparency or to the opposite faces of the transparency being non-parallel.
Heretofore aircraft windscreens have been measured while stationary and by using static test targets and photographic techniques. Several windscreen distortion, static measuring methods are described in Air Force Aerospace Medical Research Laboratory Report No. AFAMRL-TR-81-21, dated February 1981. Also, windscreens are inspected visually for distortion by inspectors that bob their heads to observe the effects of dynamic distortion, that is, distortion produced in the image of an object as viewed through the windscreen when the object and windscreen are moving relative to one another. This inspection technique, being very crude and subjective, is not amenable to standardization, nor preparation of an objective record of the degree of distortion observed by the inspector.
Consequently, a need exists for a technique to record dynamic distortion in transparencies, such as aircraft windscreens, in a way which will facilitate establishment of pass-fail type criteria for use in production inspection or comparison of windscreens.